collatz-conjecture: Add exercise to track (#593)

config.json

@@ -58,6 +58,16 @@
       "topics": [
       ]
     },
+    {
+      "uuid": "6eba4eac-8395-4ad6-ac21-3651a11ab4b4",
+      "slug": "collatz-conjecture",
+      "core": false,
+      "unlocked_by": null,
+      "difficulty": 1,
+      "topics": [
+        "Maybe"
+      ]
+    },
     {
       "uuid": "85d77b8e-9b87-4d02-9fba-81f843bd66f1",
       "slug": "rna-transcription",

exercises/collatz-conjecture/README.md

@@ -0,0 +1,87 @@
+# Collatz Conjecture
+
+The Collatz Conjecture or 3x+1 problem can be summarized as follows:
+
+Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is
+odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely.
+The conjecture states that no matter which number you start with, you will
+always reach 1 eventually.
+
+Given a number n, return the number of steps required to reach 1.
+
+## Examples
+Starting with n = 12, the steps would be as follows:
+
+0. 12
+1. 6
+2. 3
+3. 10
+4. 5
+5. 16
+6. 8
+7. 4
+8. 2
+9. 1
+
+Resulting in 9 steps. So for input n = 12, the return value would be 9.
+
+
+
+## Getting Started
+
+For installation and learning resources, refer to the
+[exercism help page](http://exercism.io/languages/haskell).
+
+## Running the tests
+
+To run the test suite, execute the following command:
+
+```bash
+stack test
+```
+
+#### If you get an error message like this...
+
+```
+No .cabal file found in directory
+```
+
+You are probably running an old stack version and need
+to upgrade it.
+
+#### Otherwise, if you get an error message like this...
+
+```
+No compiler found, expected minor version match with...
+Try running "stack setup" to install the correct GHC...
+```
+
+Just do as it says and it will download and install
+the correct compiler version:
+
+```bash
+stack setup
+```
+
+## Running *GHCi*
+
+If you want to play with your solution in GHCi, just run the command:
+
+```bash
+stack ghci
+```
+
+## Feedback, Issues, Pull Requests
+
+The [exercism/haskell](https://github.com/exercism/haskell) repository on
+GitHub is the home for all of the Haskell exercises.
+
+If you have feedback about an exercise, or want to help implementing a new
+one, head over there and create an issue.  We'll do our best to help you!
+
+## Source
+
+An unsolved problem in mathematics named after mathematician Lothar Collatz [https://en.wikipedia.org/wiki/3x_%2B_1_problem](https://en.wikipedia.org/wiki/3x_%2B_1_problem)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/collatz-conjecture/examples/success-standard/package.yaml

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+name: collatz-conjecture
+
+dependencies:
+  - base
+
+library:
+  exposed-modules: CollatzConjecture
+  source-dirs: src
+
+tests:
+  test:
+    main: Tests.hs
+    source-dirs: test
+    dependencies:
+      - collatz-conjecture
+      - hspec

exercises/collatz-conjecture/examples/success-standard/src/CollatzConjecture.hs

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+module CollatzConjecture (collatz) where
+
+collatzHelper :: Integer -> Integer -> Maybe Integer
+collatzHelper t x | x == 1    = Just t
+                  | even x    = collatzHelper (t+1) (x `div` 2)
+                  | otherwise = collatzHelper (t+1) (x*3 + 1)
+
+collatz :: Integer -> Maybe Integer
+collatz x = if x <= 0 then Nothing else collatzHelper 0 x

exercises/collatz-conjecture/package.yaml

@@ -0,0 +1,20 @@
+name: collatz-conjecture
+version: 1.1.1.1
+
+dependencies:
+  - base
+
+library:
+  exposed-modules: CollatzConjecture
+  source-dirs: src
+  dependencies:
+  # - foo       # List here the packages you
+  # - bar       # want to use in your solution.
+
+tests:
+  test:
+    main: Tests.hs
+    source-dirs: test
+    dependencies:
+      - collatz-conjecture
+      - hspec

exercises/collatz-conjecture/src/CollatzConjecture.hs

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+module CollatzConjecture (collatz) where
+
+collatz :: Integer -> Maybe Integer
+collatz = error "You need to implement this function."

exercises/collatz-conjecture/stack.yaml

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+resolver: lts-8.21

exercises/collatz-conjecture/test/Tests.hs

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+{-# OPTIONS_GHC -fno-warn-type-defaults #-}
+{-# LANGUAGE RecordWildCards #-}
+
+import Data.Foldable     (for_)
+import Test.Hspec        (Spec, describe, it, shouldBe)
+import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)
+
+import CollatzConjecture (collatz)
+
+main :: IO ()
+main = hspecWith defaultConfig {configFastFail = True} specs
+
+specs :: Spec
+specs = describe "collatz" $ for_ cases test
+  where
+
+    test Case{..} = it description assertion
+      where
+        assertion = collatz number `shouldBe` expected
+
+
+data Case = Case { description :: String
+                 , number      :: Integer
+                 , expected    :: Maybe Integer
+                 }
+
+cases :: [Case]
+cases = [ Case { description = "zero steps for one"
+               , number      = 1
+               , expected    = Just 0
+               }
+        , Case { description = "divide if even"
+               , number      = 16
+               , expected    = Just 4
+               }
+        , Case { description = "even and odd steps"
+               , number      = 12
+               , expected    = Just 9
+               }
+        , Case { description = "Large number of even and odd steps"
+               , number      = 1000000
+               , expected    = Just 152
+               }
+        , Case { description = "zero is an error"
+               , number      = 0
+               , expected    = Nothing
+               }
+        , Case { description = "negative value is an error"
+               , number      = -15
+               , expected    = Nothing
+               }
+        ]